Yes, you are being thermal throttled. I have 5870's that did this in the winter (outside rigs, frozen heatpipes = skyrocketing temperatures = throttle). You need to find a way to cool your card better by doing 1 or a combination of these things...
Clean out your fan/heatsink
Increase fan speed
Run reduced core clocks.
Run reduced core voltage.
You can also try re-greasing your heatsink, but you will need to get new thermal pads if it is a reference heatsink because just taking off the heatsink will ruin these pads.
You should also already be underclocking your memory if it is a dedicated miner. 150-200 is good for worksize 128, and 300-380 for worksize 256 (slightly faster, but slightly more electricity)
Also I should note: When your card is thermal throttling, your fan speed profile will be changed to one specific to thermal throttling as well, which is about 55%, even if you fix the fan to a different value. You HAVE to cool the card better (or cause less heat by underclocking/undervolting the gpu core) to prevent this from happening in the first place.
Undervolting can do wonders for cooling a card, but the power difference from going from 160mem to 300mem is almost negligible.
dunno about the power difference, but it makes a couple degrees C different on the temps
i lose about 1 or 2mhash going from 295 to 195